Hi,
You have a javascript error in your checkout page.
Uncaught TypeError: window.addEvent is not a function
That error means that you do not have Mootools in your website but because you have some mootools popups (squeezebox), you have that error.
Problem is that all the javascript after that line is not processed. So when you click on the button to validate the checkout, the javascript is not processed and the variable to indicate that the checkout is completed is not fill by the javascript.
Regards,